1. Field of the Invention
The present invention relates to an electric control apparatus for an ice making machine wherein an upright ice making plate is provided at its rear surface with an evaporator to be supplied with compressed refrigerant during an ice making cycle for forming ice cubes on the front surface of the ice making plate, and wherein during a defrost cycle, the evaporator is supplied with hot gas and the ice making plate is supplied at its rear surface with fresh water from an external source of water to release the formed ice cubes therefrom and drop them into an ice stocker. More particularly, the present invention relates to an improvement of defrost cycle control means in the electric control apparatus for controling the defrost cycle of the ice making machine.
2. Description of the Prior Art
As is disclosed in Japanese Utility Model Publication No. 61-42058, a conventional defrost cycle control device is designed to turn on a hot gas valve for supply of hot gas to an evaporator and a water valve for supply of fresh water to the rear surface of an upright ice making plate on start of a defrost cycle and to turn off the hot gas and water valves to end the defrost cycle when a temperature detected by a thermal sensor at the outlet of the evaporator becomes higher than a predetermined temperature. In the defrost cycle control device, a timer is also provided to measure a predetermined time when the hot gas and water valves are turned on, thereby maintaining the defrost cycle during the predetermined time even if the detected temperature becomes higher than the predetermined temperature. This is useful to ensure the supply of fresh water to be used at the ice making cycle.
In the ice making machine of this kind, however, the ice making plate is made of stainless steel; the heat transfer coefficient of this material is relatively low, and it is difficult to equalize the heat transfer coefficient between the ice making plate and the evaporator coil because of a difference in soldering. On the other hand, the temperature detected by the thermal sensor is greatly influenced by an ambient temperature and a temperature of defrost water supplied through the water valve. In a condition where both the ambient temperature and the defrost water temperature are low, the temperature detected at the outlet of the evaporator saturates at the end of the defrost cycle, resulting in a significant difference between the detected temperature and presence of the ice cubes on the ice making plate. It is, therefore, difficult to accurately detect presence of ice cubes on the ice making plate. If the temperature for detecting the end of the defrost cycle was determined to be low, the defrost cycle of operation would be ended in spite of presence of the ice cubes to be released from the ice making plate. If the temperature for detecting the end of the defrost cycle was determined to be high, the defrost cycle of operation would not be ended in spite of no presence of the ice cubes to be released from the ice making plate.